It's a bit convoluted presentation, that's for sure.
But a particular date in a year (say, Christmas or your birthday) in a 400 year period will fall on Monday 56-58 times, except for the Feb 29 which will only do so 15 times.
The best way to verify this "reading" is to do 7 x (56 to 58) = 392 to 406, or close to 400 (each row should add up to exactly 400).
The next column is about Tuesday, etc. and since they are dependent (eg. when Monday "happens", all the other days follow right after :)), by enumerating all distributions, the poster has noticed there are only 8 different possible distributions (unsurprising because of the repeated nature and 7 days in a week + leap years).
